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A metal–organic framework replete with ordered donor–acceptor
catenanesw
Qiaowei Li,a Wenyu Zhang,a Ognjen S. Miljanic,za Carolyn B. Knobler,a
J. Fraser Stoddart*band Omar M. Yaghi*
a
Received (in Austin, TX, USA) 24th September 2009, Accepted 16th November 2009
First published as an Advance Article on the web 3rd December 2009
DOI: 10.1039/b919923c
A metal–organic framework was constructed from struts in
which donor–acceptor [2]catenane units become integrated, leading
to a high density of these molecular machinery modules
positioned precisely in well-defined layered (2D) structures.
Mechanically interlocked molecules, such as bistable catenanes
and rotaxanes1 have been used as intrinsically switchable
components in molecular electronic devices2 and nanoelectro-
mechanical systems3 in the form of molecular shuttles4 and
switches.5 The performance of these devices is highly dependent
on our ability to place them in a precise location and
orientation,6 with high coverage on a surface,7 and with
reproducibility of action.3–5 We sought to overcome these
challenges by attaching the active molecules covalently onto
the organic links of metal–organic frameworks (MOFs).8 In
this way, the MOF backbone—metal-oxide joints9 and organic
struts—could be viewed as a platform onto which active
molecules may be precisely attached.10 Although MOFs
having rotaxanes and pseudorotaxanes as links have been
reported,11 catenane-MOFs remain unknown, presumably
because of the crystallization challenge presented by the sheer
size, flexibility, and asymmetry of the strut. Here, we report
the successful synthesis and crystal structure of MOF-1011 in
which a bulge-shaped strut BPP34C10DC-CAT is linked to a
trigonal Cu(I) unit to give (Fig. 1) a two-dimensional arrangement
based on the sql topology.12 This structure is replete with
ordered catenanes (one per copper unit, eight per unit cell, and
eighty-one per 100 nm2 surface) throughout the crystal.
Single crystals of Cu(BPP34C10DC-CAT)(NO3)3 (MOF-1011)
were obtained by heating H2BPP34C10DC-CAT (Fig. 1a) and
Cu(NO3)2�2.5H2O in a mixture of N,N-dimethylformamide
(DMF), EtOH, and H2O. Here, H2BPP34C10DC-CAT
was synthesized by using a biscarboxylic acid containing
bis-p-phenylene[34]-crown-10 (BPP34C10)13 to assist the formation
of a mechanically interlocking tetracationic cyclophane
cyclobis(paraquat-p-phenylene) (CBPQT4+)14 by clipping the
1,10-[1,4-phenylenebis(methylene)bis(4,40-bipyridinium) dication
with 1,4-bis(bromomethyl)benzene around the p-electron-richhydroquinone unit of the crown ether.13 The single crystal
structure of BPP34C10DE-CAT (Fig. 1b), the ester form
of H2BPP34C10DC-CAT, provided direct evidence for the
formation of the mechanically interlocked structure. The strut
is 19.3 A in length (distance between the two carboxylate
carbon atoms) with apparent bending in the linear strut (the
deviations of two C–CRC–C dihedral angles are 9.21 and
5.71, respectively), a phenomenon commonly observed15 in
strained large conjugated p-systems containing acetylenic
bonds. In this bulge-shaped strut (Fig. 1c), the farthest atom
in the catenane unit is 13.5 A away from the organic back-
bone. This strut,y with a molecular weight of 1779.46 g mol�1,
includes an unprecedented 100 non-hydrogen atoms, exclud-
ing anions, making it possibly the most complicated linker
ever exploited successfully in MOF synthesis.
Single-crystal X-ray diffraction reveals that MOF-1011z(Fig. 1d) has a 2D layer structure. Cu(II) is reduced
to Cu(I) during the reaction, resulting in electron para-
magnetic resonance (EPR) silence in the product. Each Cu+
ion is bonded to two carboxylate groups from two
BPP34C10DC-CAT and to one acetylenic bond from a third
BPP34C10DC-CAT in an Z2 fashion, a coordination geometry
also observed16 in discrete p-alkyne copper(I) complexes.
Interestingly, only one acetylenic unit from each organic strut
is coordinated to Cu(I), making the strut more bent within the
2D layer (bend angles are ca. 551 and 641, respectively, for
two struts in the asymmetric unit in the unit cell) and
asymmetric. The trigonal units Cu+ and the backbones of
BPP34C10DC-CAT are articulated into a 2D network. The
smallest repeating grid in this 2D network is composed of 44
non-hydrogen atoms (36 C, 4 O and 4 Cu). Mechanically
interlocked BPP34C10 (34-membered polyether ring) and
CBPQT4+ (28-membered cyclophane) are arranged so that
they are alternating up-and-down throughout the 2D layer
(Fig. 1e). These mechanically interlocked components are
chemically bonded to the 2D net by sharing hydroquinone
rings with the backbones. Each 44-membered grid in a 2D
sheet shares six atoms with two [2]catenanes.
The layers are packed along the b-axis in an eclipsed
geometry, with a distance of 13.6 A between every two
interdigitated sheets (Fig. 2). Noncovalent bonding inter-
actions between the crown ethers and CBPQT4+ are sustained
upon the formation of the relatively stronger Cu–O and
Cu–C bonds. Further investigation of the crystal structure of
aDepartment of Chemistry and Biochemistry, University ofCalifornia—Los Angeles, 607 Charles E. Young Drive East,Los Angeles, CA 90095, USA. E-mail: [email protected]
bDepartment of Chemistry, Northwestern University,2145 Sheridan Road, Evanston, IL 60208, USA.E-mail: [email protected]
w Electronic supplementary information (ESI) available: Synthesisand characterization of MOF-1011; elemental analysis; powderX-ray diffraction. CCDC 746167 and 746168. For ESI and crystallo-graphic data in CIF or other electronic format see DOI: 10.1039/b919923cz Current address: 136 Fleming Building, Department of Chemistry,University of Houston, Houston, TX 77204, USA.
380 | Chem. Commun., 2010, 46, 380–382 This journal is �c The Royal Society of Chemistry 2010
COMMUNICATION www.rsc.org/chemcomm | ChemComm
MOF-1011 reveals that the distance between the bipyridinium
rings and the hydroquinone units is 3.4 A, 0.2 A less than that
present in the single molecule model BPP34C10DE-CAT,
which suggests enhanced p–p stacking interactions caused by
the layered extended structure. We have noted that the (R) and
(S) enantiomers of the struts are present in a 1 : 1 ratio in the
crystal structure of MOF-1011, in which the bipyridinium
rings in the CBPQT4+ cyclophane are arranged in the same
direction with respect to the linear backbone, an orientation
different from the crystal structure of BPP34C10DE-CAT,
where the CBPQT4+ cyclophane is aligned ca. 751 with respect
to the linear strut. This co-conformational change may be
caused by the increased crowding between each neighboring
layer. MOF-1011 exhibits unprecedented ordering of
[2]catenanes in an extended solid structure. Compared to other
strategies for anchoring mechanically interlocked components
on surfaces,7,17 a higher coverage density of [2]catenanes was
achieved in MOF-1011. Eighty-one [2]catenanes are precisely
and chemically bonded on two sides of every 100 nm2
backbone net, which shows a high level of uniformity and
long-range ordering of catenanes compared to chemisorbed
layers or physisorbed layers of catenanes on surfaces. The
powder X-ray diffraction patternw of the MOF-1011 bulk
material matches the simulated pattern from the crystal structure.
The purity of the MOF-1011 was further confirmed by
elemental analysis.wIn conclusion, we have incorporated donor–acceptor
[2]catenanes successfully into a solid-state 2D network. The
synthesis of MOF-1011 not only represents an entirely new
approach to anchoring components of prototypical molecular
machines into solids that is quite different from any previously
established methods, but it also ensures higher accessibility
and processability compared to 3D bulk materials. This
structure demonstrates the first step towards the construction
of new synthetic materials with comprehensive parallel
functions. Incorporating dynamic behavior into robust
crystalline scaffolds in a highly controllable fashion would
reach the desired complexity.
Fig. 2 (a) Schematic view of MOF-1011 layer structure. Copper and
the backbones of the struts are articulated into the layered structure
based on sql topology, shown in the layer with grey grids identifying
the struts to which the catenanes are fused. The [2]catenane substructures
are chemically bonded to the backbone grids along the c direction on
both sides of each layer following zigzag patterns. (b) The layers are
packed along the b-axis in an eclipsed geometry, with two neighboring
layers shown. Catenanes from two neighboring layers interdigitate
each other along the a-axis. The CBPQT4+ rings from both layers are
aligned in an alternating fashion. Color code: MOF-1011 backbone,
grey layer with black struts; BPP34C10, red rings; CBPQT4+, blue
squares.
Fig. 1 (a) Structure formula for H2BPP34C10DC-CAT. (b) Ball-
and-stick representation of the solid-state structure of BPP34C10DE-
CAT. The strut is bent as a result of the steric hindrance from the
interlocked rings. The CBPQT4+ ring is aligned ca. 751 with respect to
the linear strut. (c) Mixed ball-and-stick and space-filling representation
of the bulge-shaped BPP34C10DE-CAT. (d) Plan view of MOF-1011
along the b-axis. The planar chirality of each catenane substructure is
labeled either (R) or (S). (e) Side-on view of MOF-1011 along the
a-axis. The [2]catenane substructures are positioned equally on both
sides of each layer. Cu, gold; crown ethers, red; CBPQT4+, blue;
organic struts, black. In (d) and (e), the CBPQT4+ rings are illustrated
in space-filling format while other atoms are portrayed with a
ball-and-stick representation. All hydrogen atoms and anions have
been omitted for clarity.
This journal is �c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 380–382 | 381
We thank Dr Saeed Kahn for his help with the crystallo-
graphy. This work was supported at UCLA by the Department
of Defense (DTRA: HDTRA1-08-10023) and is based upon
work supported at Northwestern University by the National
Science Foundation under CHE-0924620, and also by the Air
Force Office of Scientific Research (AFOSR) under their
Multidisciplinary University Research Initiative (MURI),
award number FA9550-07-1-0534.
Notes and references
y The CBPQT4+ ring, a p-electron deficient cyclophane, is tightlycatenaned by the crown ether macrocycle as the consequence ofmultiple [C–H� � �O], [C–H� � �p] and [p� � �p] stacking interactions.18
Interestingly, the [2]catenane is endowed with planar chirality19
induced within the crown ether since free rotation of the strut isprohibited around the tetrasubstituted hydroquinone ring on theaccount of the BPP34C10 ring being catenaned by the CBPQT4+
ring. Equal numbers of molecules with (R) and (S) chiralities wereidentified in the crystal structure, rendering crystals racemic overall.z Crystal data for MOF-1011 after SQUEEZE: C164H156Cu2N8O28,Mr = 2814.05, orthorhombic, space group Pca21, a = 30.5784(10),b=13.5840(4), c=32.0739(8) A,V=16808.3(8) A3,Dc= 1.112 g cm�3,l = 1.54178 A, Z = 4, R1 [I 4 2s(I)] = 0.1976, wR2 (all data) =0.4854, GOF = 1.829. The various geometrical restraints were usedinvolving some atoms of the crown ether moiety. Benzene rings wererefined as perfect hexagons to minimize the number of refinedparameters. Assignment and refinement of the structure proved theformation of the catenane-MOF.
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382 | Chem. Commun., 2010, 46, 380–382 This journal is �c The Royal Society of Chemistry 2010